1. Field of the Invention
The present invention relates to a method and apparatus for the detection of incipient fog on surfaces of various types. This invention is particularly useful for detecting incipient fog on windows or windshields of vehicles.
2. Description of the Prior Art
The personal experience of many drivers suggests that under certain conditions sudden fogging of the interior of the windshield can constitute a critical safety hazard. Such "white out" conditions are created when moist ambient air is allowed to contact the windshield surface which is at a temperature either at or below the dew point for the existing relative humidity. Naturally, if the development of dense fog is particularly sudden, the driver's vision may be severely impaired, endangering both the driver and other motorists.
Several fog detection systems have been tested and deployed in the past, but these approaches all suffer from the same disadvantage. These prior art approaches develop a fog presence signal only after fog has begun to form on the monitored surface. However, under certain conditions, dense fog can form very suddenly and the only means to clear the obscuring fog may be too slow to provide the driver with unimpaired vision at all times. This fogging problem is particularly dangerous when driving into the sun which may be at a low elevation. A low sun angle, coupled with optical scattering or diffusion from fog on the inside surface of the windshield can temporality blind the driver and put many people at risk.
Traditional defogging systems rely on either air blown at or over the fogged surface, or electric heating of the glass surface to dissipate the fog. In most vehicular applications the standard defogger for the windshield is an electric blower designed to blow warm air on the glass that is within the driver's field of view. Embedded or surface heating elements are commonly used for rear glass surfaces to accomplish defogging or de-icing. Regulatory considerations restrict the use of such electric heating grids to the rear glass, and reduce the available options for windshield defogging to either blown air or transparent electric heating films, which are very expensive and relatively fragile.
A well known problem with a blower defogger is that it is not fast acting, and they are even less efficient if warm air is not available to accelerate the removal of the condensate, i.e., fog. Such a situation is common on first startup, when the engine is cold and the heater core is not able to add the necessary heat to the defogger air stream. In fact, under some situations operating the blower can exacerbate the situation by blowing moisture laden air onto the windshield inner surface, which may be far below the dew point in temperature.
This blower induced fogging is not as common now as it once was because automotive vehicle manufacturers now partly dry the air that is blown across the windshield. Such drying is accomplished by operating the air conditioning compressor whenever the defogger is engaged. Passing ambient air over the substantially cooled evaporator coil of the A/C system "pre-condenses" much of the humidity, thereby preventing its deposition on the moderately cool glass surface. After the air has been dehumidified, it is often passed through a heat exchanger in order to improve defogging efficacy.
In essence, the problem in achieving efficient defogging in an automotive environment is the delay in activating the defogging means. That is, once fog is detected there is an essentially unavoidable delay in clearing the glass. Increasing the air flow rate over the glass and/or adding more heat to the air stream is only partly feasible because of space and power limitations. A more practical and desirable approach to the problem would be to use a predictive fog sensing method wherein the conditions leading to fog formation would be monitored and evaluated by an intelligent controller which could begin the defogging process before the onset of actual, observable fog. Such a preemptive defogging system would, in fact not be a defogger at all because with proper inputs and controls fog could never actually form. The necessity for a rapid response to air and glass temperature in addition to accurate monitoring of relative humidity makes such an approach technically difficult. Implementing such a predictive system would be difficult in an automotive setting where environmental changes are rapid and occur over a very wide temperature range.